There are machine parts, for example angle boards on lathes and hoisting cages in standard material handling systems, that are driven by means of standard electric drive devices and where the “hanging load” state can arise. The electric drive device therein has at least one electric machine that can be motor-operated and in particular also generator-operated. In the “hanging load” state a minimum moment must be exerted by the drive, meaning by the electric machine, in order to counteract the gravitational force pulling the load down. For that purpose the drive has, for example, a power converter for powering the electric machine. However, the electric machine can also be connected as a drive device directly to a supply voltage.
In the event of an outage or other fault in the electric drive device it will be unable to exert the requisite moment. If no further measures are taken, a hoisting cage, for instance, may drop and put persons and objects at risk.
As a safeguard against such risks, safety devices such as releasable brakes are known that will engage in the event of faults and power outages and put the hanging load into a safe state. For detecting faults of said kind it is possible to employ, for example, multi-channel, redundant safety systems and components that cause one or more safety devices to trigger. Safety devices of said kind are used advantageously not only when the drive device is at rest but also while it is operating.
The transition to the safe state can, of course, be initiated only if a fault having occurred is also detected. Depending on the specific implementation and application, the fault therein needs to be detected appropriately quickly. The implementation therein relates in particular to the mechanism of a machine, wherein, for instance, transmission ratios of gears have a possible impact.
There are faults that cannot be detected directly or whose detection would require complex additional measures. Examples of faults of this kind are:    breaking of the sensor shaft, meaning of the rotationally fixed link between the drive device and a sensor device (sensor) that registers an actual position or actual rotational speed of the drive device,    malfunctions in the sensor system itself that give rise to apparently correct signals.
Faults of said kind cannot be detected in the case of a single-channel sensor device. Sensor signals can, for example, assume a static state in the event of a fault, meaning that although the signals of the sensor are indeed correct they are following a movement of the drive device because, for instance, there is a fault in the sensor system or a fault in a coupling device between the sensor and the drive device. Breaking of a sensor shaft means, for example, that a frictional connection between a motor shaft and sensor shaft will have been lost. Apart from said possibility of a broken sensor shaft there are, though, other possibilities such as loss of the frictional connection between the sensor shaft and a code disk of the sensor. The sensor's code disk serves to generate sensor signals and is referred to frequently also as a sensor disk.
Examples of known sensors are location sensors, speed sensors, and acceleration sensors. For registering location, position, linear speed, and rotational speed, sensors can be used that generate two sinusoidal or square wave signals offset by 90°. The location or rotational speed can be determined from said signals. If said sensor signals become static or the sensor shaft breaks and the drive device, which is to say the electric motor, remains in the active idling state (moment is exerted against the force due to weight, rotational speed is zero), then the sensor signals (sensor variables) will freeze unnoticed. Location and rotational speed regulators would then be in an open loop mode. In particular a control loop for regulating location, speed and/or acceleration would hence be open, so that controlled operation is no longer possible. The drive device will then be in a labile state. The slightest disturbing moments could in the case of, for instance, hoisting gear then cause a load to be dropped.
While a machine is in operation, a drive device assigned thereto will continue being moved, for example, from one position to another. The assumed faults can therein be detected by observing certain controlled variables. As that is a very complex process and the risk that the sensor shaft will break or that the sensor signals will become static is assessed as being at most very slight, such additional measures for monitoring are frequently omitted.
U.S. Pat. No. 4,115,958 A discloses a monitoring device for a drive device which is provided for monitoring a movement of the drive device. The monitoring device has a first and a second sensor, the second sensor being provided for monitoring the first sensor.
U.S. Pat. No. 4,807,153 A discloses a monitoring device for a drive device which is provided for monitoring a movement of the drive device. The monitoring device has a sensor. The motor current and the terminal voltage of a motor of the drive device are also recorded. On the basis of the motor current, the terminal voltage and motor-specific characteristic variables (resistance and inductance), an estimated value for a motor velocity is determined so that the motor velocity determined by means of the sensor can be checked with regard to its validity.
EP 0 658 832 A discloses a sensor which supplies an incremental signal on one hand and an absolute signal on the other hand so that the two signals can be mutually checked with regard to their validity.
It is known from U.S. Pat. No. 6,071,477 A to connect a stepper motor to a driven shaft by means of a coupling. The rotary position of the shaft is sensed by means of an encoder.